This invention relates to frequency meter circuits for measuring the repetition rate of an input pulse train, or of any repetitive occurrence which can be made to generate a pulse, and is especially suited to measure the rate of the human heartbeat.
The two classes of frequency meter circuits most like the present device might be called "pulse averaging" and the "phase lock" types. The present device might be called a "rate matching" type.
A pulse averaging type of frequency meter generates a standard pulse, one with a fixed voltage and duration, total charge, etc., for each input pulse. The resulting train of standard pulses is then time averaged in amplitude, usually by a low pass electrical filter. The resulting averaged filter output is proportional to the input pulse rate, and is taken as the device output. Wide-range audio range frequency meters are very commonly of this type, as are most heart rate meters.
The phase lock type of frequency meter has an oscillator whose output frequency may be controlled, usually electrically, over the range of pulse input rates expected. A phase comparison circuit generates an output indicating the relative phase difference between the input pulse train and the instrument's internal oscillator. This phase output is applied as negative feedback to control the instrument's oscillator and acts to maintain a constant phase relationship between the input pulse train and the oscillator. This effectively "locks" the oscillator to the frequency of the input train. The oscillator frequency is a known (usually linear) function of the control voltage, so when the oscillator is locked, its control voltage is a measure of the input frequency. The control voltage is taken as the device output. This indirect approach has advantages in some situations and is used as the basis of nearly all deep space communications receivers.
The rate matching frequency meter of the present invention is like the phase lock type in that it is indirect, using negative feedback and taking a control voltage as its output. It is unlike a phase lock in that the controlled factor is a ramp rate, not a frequency, and in that the feedback is generated from ramp overrun. It does not contain an oscillator, while a phase lock type must.
At each input pulse a ramp is started. Here "ramp" means any quantity, such as voltage, charge, etc., which begins at some definite value and changes at some definite rate. When the next input pulse occurs, the value of the quantity at that time is compared with a preset value and the ramp is then restarted. Negative feedback typically is used to control the ramp rate, adjusting it to cause the final value of the quantity to equal the preset value. The control voltage that determines the feedback is taken as the device output, which indicates the pulse repetition rate.